Radial-piston rotary hydrostatic machines

ABSTRACT

A hydrostatic motor or pump which comprises a cylinder block with two banks of radial cylinders surrounded by a relatively rotatable cam ring affording cam tracks which coact with follower rollers carried by radially-reciprocable pistons in the cylinders. A circular leaf spring accommodated in a circumferential groove in the cylinder block marginally overlies the bodies of the pistons in the two banks being disposed between their follower rollers, and resiliently biasses the pistons radially inwardly to their innermost positions, the leaf spring acting on the pistons via interposed rollers or via radial pegs carried by the pistons. When the cylinders are unpressurised the leaf spring holds all the piston-and-roller assemblies withdrawn in their radially-inner positions clear of the cam tracks for freewheeling. When the cylinders are pressurised the pistons are moved radially outwardly, expanding the leaf spring, until their rollers engage the cam tracks.

United States Patent 1191 Green et a1.

1451 Oct. 21, 1975 RADIAL-PISTON ROTARY HYDROSTATIC MACHINES [73] Assignee: Newage Engineers Limited,

Stamford, England 22 Filed: Jan. 23, 1974 21 Appl.No.:435,955

[30] Foreign Application Priority Data Apr. 9, 1973 United Kingdom 16895/73 [52] U.S. Cl. 91/492; 267/167; 267/156 [51] Int. Cl. FOIB 13/06 [58] Field of Search 91/491, 492, 488; 267/155, 267/156, 167

1,947,585 6/1970 Germany 91/491 Primary Examiner-William L Freeh Attorney, Agent, or Firm-Cushman, Darby & Cushman [57] ABSTRACT A hydrostatic motor or pump which comprises a cylinder block with two banks of radial cylinders surrounded by a relatively rotatable cam ring affording cam tracks which coact with follower rollers carried by radially-reciprocable pistons in the cylinders. A circular leaf spring accommodated in a circumferential groove in the cylinder block marginally overlies the bodies of the pistons in the two banks being disposed between their follower rollers, and resiliently biasses the pistons radially inwardly to their innermost positions, the leaf spring acting on the pistons via interposed rollers or via radial pegs carried by the pistons.

[56] References Cited When the cylinders are unpressurised the leaf spring UNITED STATES PATENTS holds all the piston-and-roller assemblies withdrawn in 3,225,701 12/1965 Griffith 91/488 their radially-inner positions clear of the cam tracks 3,270,685 1966 Eickrflann for freewheeling. When the cylinders are pressurised l; 159 2 the pistons are moved radially outwardly, expanding 1c mann t 7 3,824,899 7/1974 Dzioba 91/491 gi z Sprmg rollers engage the cam FOREIGN PATENTS OR APPLICATIONS I 5 D 1,249,873 11/1960 France 91/491 9 C raw'ng F'gures B H 0 C 3 2 R C a F 5 E I H US. Patent Oct. 21, 1975 Sheet10f4 3,913,455

US. Patent Oct. 21, 1975 Sheet 2 of4 3,913,455

US. Patent Oct. 21, 1975 Sheet4 0f4 3,913,455

RADIAL-PISTON ROTARY HYDROSTATIC MACHINES This invention relates to positive-displacement camcontrolled rotary hydrostatic machines of the radiallyreciprocating piston and cam ring type. A machine of this type, which may be operated either as a hydrostatic motor or as a positive-displacement pump, has a cylinder block formed with one or more banks of radial cylinders in which piston assemblies reciprocate radially, the piston assemblies having cam followers, e.g. balls or rollers, which co-act with a circumferential cam profile formed on a cam ring which is co-axial with and rotatable relatively to the cylinder block. The cam ring may surround the cylinder block or vice versa, and either the cylinder block may be fixed and the cam ring rotatable about the axis of the cylinder block, or vice versa. For use as a hydrostatic motor, the inner ends of the cylinders are connected in the required sequence to a supply of pressurised hydraulic fluid and to a hydraulic return line, the resultant radial reciprocation of the pistons producing the required relative rotary drive between the cylinder block and the cam ring.

When a rotary hydrostatic machine of the type specified is rotating whilst its cylinders are unpressurised, so that there is insufficient radial bias on the pistons to keep their cam followers in rolling contact with the associated cam profile, the cam followers will intermittently hit the surface of the cam profile during such free rotation, causing noise and possibly damage. For example, in the case of a hydrostatic motor which normally provides the propulsion for a ground vehicle, this hitting of the cam surface by the piston-and-follower assemblies may occur when the vehicle is being towed whilst the motor is unpressurised.

An object of the present invention is to provide a hydrostatic motor of the type referred to above with a simple means for preventing the cam followers of the piston assemblies from hitting their associated cam surface under such conditions.

Another object of the present invention, is to provide arotary hydrostatic machine of the kind specified, with a radially-resilient circular spring, e.g. a leaf spring formed by a fiat spring strip curled lengthwise into circular form with its two end portions overlapping one another, which co-acts with the piston-and-follower assemblies of either one bank or two adjacent banks of the cylinders, and acts radially on each piston thereof to hold the piston resiliently in a radially-withdrawn position in which its follower is clear of the whole of the associated relatively-rotating cam profile.

For example in the case of a machine in which the cylinder block is surrounded by the cam ring, the or each circular spring may be a leaf spring located in a circumferential groove formed in the exterior of the cylinder block and intersecting all the cylinders of at least one bank on one side of the cylinders.

With such an arrangement one edge of the or each circular leaf spring will overlie a part of the circular body of each piston outside one end face of the cylindrical follower roller carried by the piston, so as to engage the piston radially with respect to the cylinder block, either directly or through the intermediary of a small roller or a radial peg or some other interposed member.

The invention may be carried into practice in various ways, but for the better understanding of the invention two specific embodiments of the invention will now be described by way of example only, and with reference to the accompanying drawings, in which:

FIG. 1 is a diagrammatic side view of the cylinder block of a hydrostatic motor of the type specified;

FIG. 2 is a diagrammatic view of the cylinder block and surrounding cam ring of the motor, in section through the groove P in the cylinder block but showing only two cylinders of the block;

FIG. 3 is a view in axial section of a second embodiment of hydrostatic motor, the section line being indicated at III-III in FIG. 4;

FIG. 4 is a cross-section on the line IVIV of FIG. 3, showing the machine with its pistons retracted; and

FIG. 5 is a fragmentary view showing the rotatable central pintle and porting arrangements of the motor.

In the first illustrated embodiment, FIGS. 1 and 2 shows schematically a hydrostatic motor which comprises a cam ring A formed with an internal endless circumferential cam profile Al and surrounding coaxially a cylinder block B formed with two banks each of six radial cylinders, l to 6 and 1', 2', etc. The cam ring profile Al is shown purely diagrammatically in FIG. 2, no attempt having been made to illustrate its exact configuration. The cylinder block B is formed with a circumferential groove P in which is located a circular leaf spring F, whose two ends F1 and F2 overlap one another to allow the resilient radial expansion and contractionbf the spring. The groove P intersects the cylinders of the two adjacent banks on their adjacent sides as shown, to a radial depth insufficient to open into the inner ends of the cylinders when their pistons are at their radially-outermost positions at the ends of theiroutward strokes. The piston C themselves carry cylindrical follower rollers D, each roller D' being journalled in apart-cylindrical bearing shell G whose end portions, backed by opposed skirt portions H of the piston, extend'around somewhat more than of the circumference of the respective roller D so that the roller is trapped'in' its journal shell and cannot escape radially from the piston. This feature is the subject of our co-pending US; Pat. Application Ser. No. 388622.

As shown in FIG. 1, the skirt of each piston C is cut away at H1 on either side up to the corresponding end face of its rol-lerD; and each shell G is correspondingly cut away, to accommodate the overlapping edge portion of the circular leaf spring F, and a small roller E is trapped between the margin of the leaf spring F and the margin of the concave surface of the bearing shell G at the base of the cut-away side of the piston, so that the spring F canfact radially inwardly on each pistonC via the roller E to bias the piston radially inwardly and to tend to hold the piston resiliently in its innermost radial position, in which the follower roller D is clear of the path of the peaks of the cam profile A1 as the latter rotates relatively to'the cylinder block.

Conventional rotary valve arrangements are provided for connecting the inner ends of the cylinders cyclically, via passages R, to the fluid pressure inlet and return ports (not shown) of the motor.

' Thus so long as the cylinders remain unpressurised, the leaf springF acting radially-inwardly on all the pistons C via their trapped rollers E will retain the pistonand-follower assemblies in their radially-withdrawn positions, as shown in firm lines for all the cylinders l to 6 in FIG. 2, so that the cam ring A can be freely rotated relatively to the cylinder block B without the rollers D hitting the rotating cam profile A1.

When the cylinders are pressurised however, the leaf spring F will yield resiliently to allow the pistons to be moved radially-outwardly by the fluid pressure acting on 'them,'to positions in which their rollers'D engage and drive the cam, as shown at C, D in the case of the cylinder 2in FIG. 2. The spring yields by radial expansion, its overlapped end portions sliding circumferentially relatively to one another. When the pressure in the cylinders'falls back to zero, the leaf spring F con-' tracts radially to withdraw all the piston-and-follow'er assemblies once more to their innermost positionsclear of the path of the relatively-rotating cam.

As seen in FIG. 1, the leaf spring co-operates with'the pistons of two adjacent banks of cylinders, opposite edges of the spring acting resiliently on all the cylinders in the respective banks.

The spring F'need not act on each piston via a small trapped or journalled roller, but might bear directly against a surface of the body of the piston itself;

It will'be clearthat in the case of a motor having an I external cylinder block surrounding an internal cam ring with an outwardly-facingcam surface, the leaf spring would be,arranged in aninternal groove in the cylinder block to bear resiliently outwardlyon the piston-and-follower assemblies, so as to retract them radially-outwardly away from the cam when the cylinders were not pressurised.

FIGS, 3 to 5 show in greater detail another construction of hydrostaticmotor embodying the presentinvention, the motor being suitable for the direct drive of a vehicle road or land wheel, and comprising a nonrotatable cylinder block 11 bolted to a back plate 12, by means of bolts 13, and a rotatable hub assembly 14 providing a wheel ,flange 15 to which a wheel can be bolted, the hub assembly 14 being rotatably mounted in bearings 16, 17 on the back plate 12 and cylinder.

block 11 and including aomotor housing 18 which encloses the fixed cylinder block 11. The cylinder block 11 isformed with twelve radial cylinders 20, arranged in two axially-spaced. banks each of six cylinders at 60 spacings, the cylinders 20 being open at their radiallyouter ends and each being provided with an inlet and discharge port 21 at its inner end which co-operates with aring of ports formed in a pintle 25 rotatable with the hub assembly 14in a coaxial central bore 26 in the cylinder block 11, whereby the cyclic admission and discharge of hydraulic fluid to'and from the cylinders is controlled in known 'manner as the hub assembly rotates. The pintle 25 is a close fit in the bore 26 in which it rotates'sthe pintle being keyed at 27 to a central spigot block 28 bolted by bolts 29 to a dished end cover plate 30 of the hub assembly 14. The cover plate 30 is bolted by bolts 31 to the rotary motor housing 18 of the hub assembly, and its spigot block 28 is journalled by means of the bearing 17 in one end of the fixed cylinder block 11. The pintle 25 thus rotates with the hub assembly 14 and in the bore 26 in the cylinder block. The pintle 25 is formed with two axially-spaced rings of ports, 32A, 32B and 33A, 333 which respectively cooperate with the ports 21 of the two banks of cylinders 20. T he ports 32A and 32B alternate in their ring and are respectively connected to an inlet gallery 34 and to associated discharge galleries 35, all formed longitudinally in the pintle 25. The ports 33A and 33B also alternate in their ring and are respectively connected to the galleries 34 and 35, as shown. The central inlet gallery 34 leads into a circumferential groove 36 in the pintle surface, the groove 36 being in communication with a passage 37 formed in the back plate 12 and leading from an inlet port 38 for connection to a hydraulic pressure supply line, as shown in FIG. 5. The four discharge galleries 35 all lead into a common circumferential groove 39 formed in the pintle surface, the groove 39 being in communication with a discharge passage 40 formed in the back plate 12 and leading to a discharge port 41 for connection to a hydraulic fluid return line. Thus as the hub assembly 14 and the pintle 25 rotate relatively to the back plate 12 and cylinder block 1 l, the cylinder ports 21 are successively brought into alignment with the ports 32A, 32B, or 33A, 33C in the pintle and are thereby successively connected to the hydrostatic supply pressure and to the return pressure.

jsl'idablein each cylinder 20 is a cylindrical piston 50 formed in its outer end face with aconcave recess 51 lined by apart-cylindrical bearing shell 52 whose axis extends parallel tothe axisofrotation 53 of the hub. Journalled in the bearing 52 in each piston 50 is a cylindrical roller 55, part of whose cylindrical surface protrude's'outwardly from the piston bearing shell 52 and co-acts with a circumferential cam track 56 formed internally in the motor housing 18 for rotation with the hub assembly 14, and surrounding the cylinder block 1 1. There are two such cam tracks 56, spaced apart axially and co-acting with the rollers 55 of the pistons 50 of the respective two banks of cylinders. Each bearing shell 52 has'a small lubrication port in its base which communicates via a passage 58 (FIG. 3) through the recessed piston crown with the interior of the associated cylinder 20, whereby pressure fluid from the cylinder 20 is fed to the bearing 52 to lubricate its bearing surface.

The pistons 50 are biassed radially inwardly in the associated cylinders 20 by a circular leaf spring which lies in a circumferential groove 71 formed in the cylinder block 11, and in addition fluid at low hydraulic pressure fills the interior of the housing 18 and acts on the pistons 50 to bias them radially inwardly. The rollers 55 are positively retained in their bearings 52 and prevented from separating radially from the pistons by the shape of the bearings themselves. Thus each bearing shell 52 subtends an angle of about 245 at the axis of the' roller 55, so that the opposite side portions of the bearing shell 52 extend partially around the outer semicylindrical isurface of the roller 55, as shown in the" drawings. The spacing between the longitudinal edges 59 of the bearing. shell 52 is thus less than the diameter of the roller 55 which is therefore trapped against leaving the shell in the radially-outward direction. The skirts 60 of the piston 50 are similarly formed to extend beyond the level of the axis of the roller, in the outward direction of piston reciprocation, and are inwardlycurved at their outwardly-extending portions to back up-, and'support the bearing shell 52 where this wraps around the roller 55. g v v In this'manner the followerrollers 55 are positively retained radially in their bearings and are prevented from leaving th'e pistons 50 during free-wheeling of the motor. i

The follower rollers 55 are also provided with positive end location means to hold them centralised in -their bearings with respect to the axes of their respective cylinder bores and square to the-fcam tracks 56.

This end location meanscomprises two outer guide rings 65 and 67 which coaxially surround the cylinder block 11, and arelocated immediately adjacent to the outer ends of the rollers 55 of the respectivecylinder banks, the leaf spring 70 being interposed. between the cylinder banks as a spacer between the two sets of rollers 55. The outer two guiderings 65 and 67 are axially retained bymeans of spring circlips 68'fitted outside them in shallow circumferential grooves in the cylinder block, to hold the guide rings 65 and 67 in positions slightly overlapping the open ends of thecylinders 20. The guide rings 65 and 67 area free running fit on the cylinder block -11,,so that they'are freely rotatable to reduce rubbing losses between them and the rollers'by reducing the relative speeds of sliding between the rolltwill be seenfrom FIG. 3 that the circuferential I groove 71 formed in the cylinder block 11 intersects the cylinders 20, the walls of the groove 71 being'approximately coplanar with the respectiveinner side faces of the rollers 55 of the two banks'of cylinders.

' The leaf spring 70 is a'free fit in the groove7l and thus overlaps the margins of the full-diameter cylindrical bodies of the pistons 50. The intersection of the partcylindrical transverse bore 51 in each piston which provides the recess for its follower roller 55 and bearing liner shell 52, with the cylindrical outer surface of the -bo dy of the piston 50 produces a waisted profile of the two skirt portions 60 of the piston, as shown in elevation in FIG. 3, whereby one longitudinal margin of the leaf spring can partially overlie the crown of each piston 50 as shown. As also clearly shown in FIG. 3 the axial length of each follower roller 55 is less than the full diameter of the piston 50 which carries it, and the leaf spring 70 which as mentioned is a free running fit in the groove 71 lies between the follower rollers 55 of the two banks of cylinders, with the opposite edges of the leaf spring lying in opposed relationship to the adjacent end faces-of the rollers 55 of the respective banks of-cylinders, so that the edges of the leaf spring constitute abutments to provide endwise location of the follower rollers of the respective banks in the axial direction towards one another. Endwise location of the follower rollers in each bank in the direction away from the other bank of cylinders is provided by the respective rotatable guide ring 65 or 67. A peg 72 having a domed upper end is mounted in a small hole 73 formed in the body of each piston 50 at one outer end of the transverse bore 51 and at the radially-lowest part of that bore 51, the peg 72 extending parallel to the axis of the piston 50 in the radially-outward direction with respect to the cylinder block, so that the domed outer end of each peg engages the inner side of the leaf spring 70. The pegs 72 thus take the places of the small rollers E of the preceding embodiment, so that when there is no supply pressure in the inner ends of the cylinders the leaf spring 70 will act resiliently on the pistons'50 via their pegs 72 and will hold the pistons in their fullywithdrawn radially-innermost positions in the cylinders 11, as shown in FIG. 4, in which positions the rollers 55 are just clear of the cam tracks 56, and the motor housing 18 together with thec-am tracks 56 can free-wheel relatiyely to the cylinder block ll without the rollers SS-hitting'thecam tracks. 7 i v t However, when the inner ends of the cylinders 11 are pressurised for driving the motor, the supply pressure will force the pistons 50 radially outwardlyuntil their rollers 55 engage the cam tracks 56, the. leaf spring 70 yielding by expansion under the radial-pressure applied to it by the pegs 72 carried by the pistons 50, as shown in FIG. 3. When the pressure in all the cylinders is again relieved for free-wheelin'g,the leaf spring 70 again contracts to withdraw all the pistons 50. and-their follower rollers move once more to their innermost position just clear of the path of the relatively-rotating cam tracks.

It will be understood that whilst in the embodiments described and illustrated the cylindrical surfaces of the rollers and their bearing shells are generated by straight lines, it would also be possible vfor the rollers to be slightly barrel-shaped, i.e. generated by curved generators, and for thecam tracks and possibly also the bearing shells to be correspondingly curved. The terms cylindri'cal an'd part-cylindrical as'used herein are to l be interpreted accordingly.

What we claim as our invention'and desire to secure by Letters Patent is:

l. A positive-displacement cam-controlled rotary hy drostatic pressure machine comprising a cylinder block having a bank "of angularly-spaced radial cylinders,

radially-reciprocable pistons'in the cylinders, a cam ring mounted coaxially with the cylinder block for rotation relative thereto, an endless circumferential cam track on the cam ring, the pistons carrying cam followers which coact with the cam track to relate'thecyclic 45 radially-withdrawn position in which its follower is clear of the path of the cam track during rotation of the cam ring relative to the cylinder block with the cylinders unpressurized.

2. A motor as claimed in claim 1 in which the follower of each piston comprises a cylindrical roller rotatably mounted in a recess formed in the end of the piston nearest to the cam ring, the roller being prevented from separating radially from the piston, and in which the longitudinal edges of the leaf spring lie in opposed relationship to end faces of the rollers of the respective banks of cylinders, whereby the opposite edges of the leaf spring constitute abutments which provide endwise location for the follower rollers of the respective banks of cylinders.

3. A rotary machine as claimed in claim 1 in which there are two of the said banks of cylinders in the cylinder block, the cylinders in the respective banks being angularly-staggered in relation to one another, and in which the leaf spring is interposed between the two banks of cylinders with the opposite circumferential marginal portions of theleaf spring respectively overlapping the cylinders ofthe two banks and acting on the respective pistonstherein.

4. A rotary machine as claimed in claim 3 in which the cam ring circumferentially surrounds the cylinder block and the leaf spring is located as a free fit in a circumferential groove inthe outer circumference of the cylinder block, the groove intersecting the cylinders of both banks, and in which the follower of each piston comprises a cylindrical roller rotatably mounted in a recess in the end of the piston nearest to the cam ring, the roller being prevented from separating radially from the piston, and in which the longitudinal edges of the leaf lie in opposed relationship to end faces of the rollers ofthe respective banks of cylinders, whereby the opposite'edges of the leaf spring constitute abutments which provide endwise location for the rollers of the respective banks of cylinders.

5. A rotary machine as claimed in claim 4 including spacer rollers freely interposed between the inner face of the leaf spring and each piston, the axes of the spacer rollers being parallel to the axis of the cam ring, and the resilient force of the leaf spring being transmitted radially to each piston through its associated spacer roller. 6. A rotary machine as claimed in claim 4 including 7 a radial peg mounted on each piston to protrude raditrack on the cam ring facing the cylinder block, the pistons carrying followers which co-act with the cam track to convert cyclic radial reciprocating motion of the pistons in the cylinders'into continuous rotary motion of the cam ring relative to the cylinder block, inlet and exhaust ports for high pressure fluid, valve means cyclically controlling the admission of pressurized hydraulic fluid from the inlet port to the cylinders and the discharge of the fluid from the cylinders to the exhaust port in synchronism with the rotary motion of the cam ring relatively to the cylinder block for driving the motor, and a circular leaf spring comprising a flat spring strip curled lengthwise into circular form with its two end portions overlapping one another, the leaf spring being housed in an external circumferential groove in the cylinder block, which groove is positioned between the two banks of cylinders and intersects the cylinders of both banks, and in which the leaf spring overlaps the cylinders of both banks and opposite circumferential marginal portions of the leaf spring respectively act resiliently and radially-inwardly on the pistons in the respective banks, so that the leaf spring holds each piston resiliently in a radially-withdrawn inner position in which its follower is clear of the'path of the cam track during rotation of the cam ring relative to the cylinder block when the cylinders unpressurized.

8. A motor as claimed in claim 7 including spacer rollers freely interposed between the inner face of theleaf spring and each piston, the axes of the spacer rollers is transmitted to each piston through its associated peg. 

1. A positive-displacement cam-controlled rotary hydrostatic pressure machine comprising a cylinder block having a bank of angularly-spaced radial cylinders, radially-reciprocable pistons in the cylinders, a cam ring mounted coaxially with the cylinder block for rotation relative thereto, an endless circumferential cam track on the cam ring, the pistons carrying cam followers which coact with the cam track to relate the cyclic radial reciprocation motion of the pistons to rotary motion of the cam ring relative to the cylinder block, valve means cyclically controlling the inlet and discharge of hydraulic fluid to and from the cylinders in synchronism with the rotary motion of the cam ring relative to the cylinder block, and a radially-resilient circular leaf spring comprising a spring strip curled lengthwise into circular form with its two ends overlapping one another, the spring acting resiliently and radially on the pistons in the bank to hold each piston resiliently in a radially-withdrawn position in which its follower is clear of the path of the cam track during rotation of the cam ring relative to the cylinder block with the cylinders unpressurized.
 2. A motor as claimed in claim 1 in which the follower of each piston comprises a cylindrical roller rotatably mounted in a recess formed in the end of the piston nearest to the cam ring, the roller being prevented from separating radially from the piston, and in which the longitudinal edges of the leaf spring lie in opposed relationship to end faces of the rollers of the respective banks of cylinders, whereby the opposite edges of the leaf spring constitute abutments which provide endwise location for the follower rollers of the respective banks of cylinders.
 3. A rotary machine as claimed in claim 1 in which there are two of the said banks of cylinders in the cylinder block, the cylinders in the respective banks being angularly-staggered in relation to one another, and in which the leaf spring is interposed between the two banks of cylinders with the opposite circumferential marginal portions of the leaf spring respectively overlapping the cylinders of the two banks and acting on the respective pistons therein.
 4. A rotary machine as claimed in claim 3 in which the cam ring circumferentially surrounds the cylinder block and the leaf spring is located as a free fit in a circumferential groove in the outer circumference of the cylinder block, the groove intersecting the cylinders of both banks, and in which the follower of each piston comprises a cylindrical roller rotatably mounted in a recess in the end of the piston nearest to the cam ring, the roller being prevented from separating radially from the piston, and in which the longitudinal edges of the leaf lie in opposed relationship to end faces of the rollers of the respective banks of cylinders, whereby the opposite edges of the leaf spring constitute abutments which provide endwise location for the rollers of the respective banks of cylinders.
 5. A rotary machine as claimed in claim 4 including spacer rollers freely interposed between the inner face of the leaf spring and each piston, the axes of the spacer rollers being parallel to the axis of the cam ring, and the resilient force oF the leaf spring being transmitted radially to each piston through its associated spacer roller.
 6. A rotary machine as claimed in claim 4 including a radial peg mounted on each piston to protrude radially outwardly towards the leaf spring, each peg having a domed outer end in contact with the inner face of the leaf spring whereby the resilient force of the leaf spring is transmitted to each piston through its associated peg.
 7. A positive-displacement cam-controlled rotary hydrostatic motor comprising a cylinder block having two axially-spaced banks of angularly-spaced radial cylinders, the cylinders in the respective banks being axially-staggered in relation to one another, radially-reciprocal pistons in the cylinders, a cam ring coaxially surrounding the cylinder block and mounted for rotation relative thereto, an endless circumferential internal cam track on the cam ring facing the cylinder block, the pistons carrying followers which co-act with the cam track to convert cyclic radial reciprocating motion of the pistons in the cylinders into continuous rotary motion of the cam ring relative to the cylinder block, inlet and exhaust ports for high pressure fluid, valve means cyclically controlling the admission of pressurized hydraulic fluid from the inlet port to the cylinders and the discharge of the fluid from the cylinders to the exhaust port in synchronism with the rotary motion of the cam ring relatively to the cylinder block for driving the motor, and a circular leaf spring comprising a flat spring strip curled lengthwise into circular form with its two end portions overlapping one another, the leaf spring being housed in an external circumferential groove in the cylinder block, which groove is positioned between the two banks of cylinders and intersects the cylinders of both banks, and in which the leaf spring overlaps the cylinders of both banks and opposite circumferential marginal portions of the leaf spring respectively act resiliently and radially-inwardly on the pistons in the respective banks, so that the leaf spring holds each piston resiliently in a radially-withdrawn inner position in which its follower is clear of the path of the cam track during rotation of the cam ring relative to the cylinder block when the cylinders unpressurized.
 8. A motor as claimed in claim 7 including spacer rollers freely interposed between the inner face of the leaf spring and each piston, the axes of the spacer rollers being parallel to the axis of the cam ring and the resilient force of the leaf spring being transmitted to each piston through its associated spacer roller.
 9. A motor as claimed in claim 7 including a radial peg mounted on each piston to protrude radially outwardly towards the leaf spring, each peg having a domed outer end in contact with the inner face of the leaf spring whereby the resilient force of the leaf spring is transmitted to each piston through its associated peg. 